1. Technical Field
The present invention relates to a technology for light source devices and image display apparatuses, and in particular for light source devices used in image display apparatuses for displaying images by scanning a light beam.
2. Related Art
In recent years, developments of projectors related to super-high image quality, downsizing, and reduction in power consumption have been in progress from a viewpoint of creating added value. Further, in association with high power and multi-color semiconductor lasers, developments of projectors and displays using laser sources have been activated. In comparison with the UHP lamps having been used as the light sources of projectors in the past, the laser sources have advantages of high color reproducibility, capability of quick lighting, long life, and so on. Further, the light sources have high luminous efficiency, can be composed of smaller number of optical elements with reduced energy loss, and therefore, are also suitable for low power consumption.
As the image display apparatus using the laser source, there can be cited an apparatus of scanning the laser beam, which is modulated in accordance with an image signal, in two-dimensional directions. In general, in order for displaying an image by scanning a laser beam, there is adopted a configuration in which a plurality of colored beams respectively emitted from different laser sources is combined into a single light beam, and a scanning optical system scans the single light beam (see, e.g., JP-A-2003-21800 (Document 1)). For example, as shown in FIG. 5 of the Document 1, by using a dichroic mirror as a parallel plate, a plurality of colored beams can be combined with a simple configuration. The laser beams are input to the dichroic mirrors, each of which is disposed at an angle of 45 degrees with the principal ray of the laser beam, from two directions, thereby combining the colored beam transmitted through the dichroic mirror and the colored beam reflected by the dichroic mirror with each other.
The laser sources emit laser beams slightly converged so as to form beam waists on the irradiated surface, for example, taking the influences of spread of the laser beams due to the diffraction into consideration. Thus, a spot having the area corresponding to that of the pixel is formed on the irradiated surface distant from the scanning optical system. If a diverging laser beam emitted from the laser source or a laser beam collected by a light collection optical system is transmitted through the parallel plate disposed at a tilt, the astigmatic difference is increased. The astigmatic difference denotes the distance between imaging positions in the case in which the imaging positions are shifted from each other in two directions perpendicular to each other. The increase in the astigmatic difference causes a problem of making it difficult to form a spot with a desired diameter on the irradiated surface, and to accurately adjust the focusing position to degrade the image quality.